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1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to a system that recovers an optical clock signal.
2. Description of the Prior Art
Fiber optic communications require synchronization between the transmitter and receiver to derive the optical data. Fiber optic communication systems use an optical clock signal for this synchronization. The transmitter embeds the optical clock signal into an optical data signal. The receiver then extracts the optical clock signal from the optical data signal.
Fiber optic communication systems recover the clock signal from an optical domain, an electrical domain, or a combination of the optical and electrical domain. In the optical domain, the communication system recovers the optical clock signal directly from the optical data signal without any optical to electrical conversions. In the electrical domain, the communication system converts the optical data signal to an electrical data signal using an optical detector. The communication system then produces the electrical clock signal from the electrical data signal. In the combination of the optical and electrical domain, the communication system typically produces the electrical clock signal directly from the optical data signal without converting the optical clock signal to electrical. A solution to recover the optical clock signal in the optical domain with no electronic filtering of the optical clock signal is more transparent and advantageous.
In wavelength division multiplexing (WDM), the transmitter transmits different optical data signals at different wavelengths within the same fiber. Each optical data signal has the ability to carry an optical clock signal in WDM. Also, the data rates may vary between each wavelength in WDM.
Some prior systems recover the clock signal by injecting the optical data signal into a mode-locked fiber ring, a multi-segment mode-locked semiconductor laser, or a self-pulsating laser diode. However, each of these prior systems is data rate dependent and extracts the optical clock signal from only one wavelength.
Another system injects the optical data signal into an optical tank circuit. The optical tank circuit filters the data portion of the optical data signal leaving only the optical clock signal. Most optical tank circuits are data-rate and wavelength dependent. One exception is Brillouin tank circuits.
FIG. 1 depicts a prior solution using a Brillouin tank circuit that recovers an optical clock signal from the optical data signal. A laser transmitter 102 transmits an optical signal to a first modulator 104. The first modulator 104 modulates the optical signal with data into an optical data signal in a return-to-zero format. An Erbium-doped fiber amplifier (EDFA) 106 then amplifies the optical data signal. A coupler 110 receives and splits the optical data signal into a first optical data signal and a second optical data signal. The coupler 110 propagates the first optical data signal clockwise and the second optical data signal counter-clockwise in opposite directions around a fiber loop 150.
The coupler 110 transmits the first optical data signal clockwise to an isolator 120. The isolator 120 isolates the first optical data signal from the second optical data signal to prevent any signals from returning to the coupler 110. A first polarization controller 122 adjusts the polarization of the first optical data signal and transfers the first optical data signal to a second modulator 124. The polarization controllers 122, 126, and 130 align the polarizations of the optical data signals so the first optical data signal and the second optical data signal will interact. The second modulator 124 modulates the first optical data signal at 10.9 GHz. A second polarization controller 126 adjusts the polarization of the first optical data signal and transfers the first optical data signal to a 2 Kilometer polarization maintaining fiber 128.
The coupler 110 propagates the second optical data signal counter-clockwise in the direction of a third polarization controller 130. The third polarization controller 130 adjusts the polarization of the second optical data signal and transfers the second optical data signal to the fiber 128.
In the fiber 128, the first optical data signal and second optical data signal interact in an effect known as stimulated Brillouin scattering. This effect acts as an active filter and amplifies the signals that are above a Brillouin power threshold in the first optical data signal. The amplification occurs through a kinetic energy transfer from the second optical data signal to the fiber 128 and from the fiber 128 to frequency components of the first optical data signal that are above the Brillouin power threshold. The optical clock signal in the first optical data signal is above the Brillouin power threshold. Therefore, the Brillouin scattering effect amplifies the optical clock signal. Thus, the coupler 110 receives the optical clock signal from the fiber 128 and transmits the optical clock signal over an outgoing link 140.
The problem with this Brillouin optical tank system is the system does not operate for WDM signals. There is not enough gain in the EDFA 106 to be effective for WDM signals. There is a need for a system that recovers the optical clock signals from the optical data signals from each wavelength that are at different data rates.
The invention solves the above problem by recovering an optical clock signal from an optical data signal. A clock recovery system splits the optical data signal into a first optical data signal and a second optical data signal. The clock recovery system then transmits the first optical data signal and the second optical data signal in opposite directions around a fiber loop. In the fiber loop, the clock recovery system modulates and amplifies the first optical data signal to generate a modulated-amplified first optical data signal. The clock recovery system then recovers the optical clock signal after the modulated-amplified first optical data signal and the second optical data signal interact in the fiber loop.
In some embodiments of the invention, the clock recovery system aligns the polarization of the modulated-amplified first optical data signal and the second optical data signal. Also, in one embodiment of the invention, the optical clock signal is above the Brillouin power threshold. In some embodiments of the invention, the optical data signal is an optical WDM data signal.
One advantage of the invention is that the clock recovery system is data rate independent. Thus, the clock recovery system can recover clock signals at different data rates. Another advantage is the clock recovery system is wavelength independent. The clock recovery system recovers clock signals from different wavelengths. Therefore, the invention can recover clock signals from WDM signals, which have different clock signals at different wavelengths with different data rates.